X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. X-ray reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, in the vicinity of the total external reflection angle of the sample material. An X-ray detector, which may comprise a detector array, senses the reflected X-rays. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. Exemplary systems and methods for XRR are described in U.S. Pat. Nos. 5,619,548, 5,923,720, 6,512,814, 6,639,968, and 6,771,735, whose disclosures are incorporated herein by reference.
The spot size and angular extent of the X-ray beam that is incident on the sample surface affect the spatial and angular resolution of XRR measurement results. In order to control these factors, U.S. Pat. No. 6,639,968, for example, provides a dynamic knife edge and shutter interposed in the X-ray beam. For measurements at low incidence angles, the knife edge is lowered very near to the surface, intercepting the incident X-ray beam and thus shortening the lateral dimension of the spot on the surface. (In the context of the present patent application and in the claims, the dimension of the spot in the direction along the surface that is parallel to the projection of the beam axis on the surface is referred to in the conventional manner as the lateral dimension, while the dimension in the direction perpendicular to the beam axis is referred to as the transverse dimension.) For high-angle measurements, at which the dynamic shutter is used, the knife edge may be raised out of the way, to allow the full intensity of the X-ray beam to be used. As another example, U.S. Pat. No. 6,771,735 uses two “gates” for blocking certain parts of the X-ray beam.
U.S. Pat. No. 6,895,075, whose disclosure is incorporated herein by reference, describes a system that combines XRR with small-angle X-ray scattering measurement (SAXS). The system uses the dynamic knife edge and shutter of U.S. Pat. No. 6,639,968 for controlling the incident beam in the vertical direction (perpendicular to the surface of the sample), together with a slit for limiting the transverse dimension of the beam in the horizontal direction. The minimum slit width is said to be about 100 μm.
U.S. Patent Application Publication 2006/0062351, whose disclosure is incorporated herein by reference, describes another multifunction X-ray analysis system, which combines XRR with SAXS and X-ray diffraction (XRD) measurement. In one embodiment, shown in FIG. 5 of this publication, a knife edge is made of a cylindrical, X-ray absorbing material, such as a metal wire. This arrangement is said to permit the lower edge of the knife to be placed very close to the surface of the sample, on the order of 3 μm above the surface, without risk of damaging the sample. The wire can be aligned with the surface accurately and thus provides a small gap above the surface whose effective height is uniform over the entire angular range of interest, typically 0-4°. Based on this example, it will be understood that in the context of the present patent application and in the claims, the term “knife edge” refers to any type of straight edge (not necessarily very sharp) that is positioned near the surface of a sample in order to create this a gap between the knife edge and the surface and to block X-rays outside the gap.